1. Field of the Invention
This invention relates to a driving method and apparatus for a plasma display panel, and more particularly to a method of generating an optimal light-emission pattern for a plasma display panel in order to select a light-emission pattern where a moving picture pseudo contour noise is minimized. Also it relates to a method of measuring a contour noise for a plasma display panel in order to rapidly calculate a contour noise degree and a method of selecting a gray scale in order to select a sub-field array and a gray scale with which the contour noise is minimized.
2. Description of the Related Art
Generally, a plasma display panel (PDP) makes a fluorescent body radiate by using an ultraviolet with a wavelength of 147 nm generated upon discharge of an inactive mixture gas such as He+Xe, Ne+Xe or He+Ne+Xe gas, to thereby display a picture including characters and graphics. Such a PDP is easy to be made into a thin-film and large-dimension type. Moreover, the PDP provides a very improved picture quality owing to a recent technical development.
Referring to FIG. 1, a discharge cell of a three-electrode, AC surface-discharge PDP includes a scanning/sustaining electrode Y and a common sustaining electrode Z provided on an upper substrate 1, and an address electrode X provided on a lower substrate 4.
The address electrode X perpendicularly cross a sustaining electrode pair including one scanning/sustaining electrode Y and one common sustaining electrode Z. On the upper substrate 1, a dielectric layer 2 and a protective film 3 are disposed in such a manner to cover the scanning/sustaining electrode Y and the common sustaining electrode Z. A dielectric layer 5 is entirely deposited onto the lower substrate 4 in such a manner to cover the address electrode X, a barrier rib 6 is provided thereon in a direction parallel to the address electrode X. A discharge such as an inactive mixture gas is injected into a discharge space defined between the upper/lower substrate 1 and 4 and the barrier rib 6.
In such a PDP, for implementing a gray level of a picture, one frame is divided into a plurality of sub-fields, each of which a brightness weighting value is given to, so as to be driven in a manner of time division. A sub-field array is defined as a set of a plurality of sub-fields which are included within one frame interval. Each sub-field included in the sub-field array is again divided into a reset interval or setup interval for initializing cells of the entire screen, an address interval for selecting cells and a sustaining interval determined in proportion to the brightness weighting value where a discharge frequency is set in advance.
FIG. 2 represents an eight bit default code including 8 sub-fields corresponding to each bit of eight bits in a sub-field array. In the eight bit default code, eight sub-fields each has the brightness weighting value increased in the order from a least significant bit to a most significant bit by 2n (wherein n=0, 1, 2, 3, 4, 5, 6 and 7) to be capable of expressing 256 gray levels.
The PDP may generate a pseudo contour noise from a moving picture because of its characteristic of implementing a gray scale of a picture by a combination of sub-fields. Hereinafter, such a moving picture pseudo contour noise is referred briefly to as a xe2x80x98contour noisexe2x80x99. If the contour noise is generated, then a pseudo contour emerges on the screen to deteriorate a display quality of moving picture. For instance, when the screen is moved to the right at a speed of 1 pixel/frame after the left half of the screen was displayed by a gray level value xe2x80x98127xe2x80x99 and the right half of the screen was displayed by a gray level value xe2x80x98128xe2x80x99 as shown in FIG. 3 and FIG. 4, an eye of an observer follows such a motion of the screen to simultaneously view lights irradiated from the adjacent two pixels. Since light-emissions from the two pixels each displaying gray levels xe2x80x98127xe2x80x99 and xe2x80x98128xe2x80x99 are accumulated at an interface between gray levels, the eye views the two pixels more brightly rather than recognizing a real brightness of the two pixels respectively. In other words, the eye views a peak white, that is, a white band emitted more brightly than the other area from the two pixels emitted by the gray levels of xe2x80x98127xe2x80x99 and xe2x80x98128xe2x80x99. On the contrary, if the screen, the left half of which is displayed by the gray level value xe2x80x98128xe2x80x99 and the right half of which is displayed by the gray level value xe2x80x98127xe2x80x99, is moved to the right, then a black band emerges from a boundary portion between gray level values xe2x80x98127xe2x80x99 and xe2x80x98128xe2x80x99.
Strategies for eliminating such a contour noise include a scheme of dividing one sub-field to add 1 or 2 sub-fields for increasing the total number of sub-fields, a scheme of re-arranging a sequence of sub-fields, a scheme of adding sub-fields and re-arranging a sequence of sub-fields, and etc. Further, they include a scheme of carrying out an error diffusion method together with any one of the above-mentioned schemes. However, since said addition of sub-fields causes a lack of an address interval or a sustaining interval, there is raised a problem that a screen becomes dark.
An example of said scheme of re-arranging sub-fields is a scheme of arranging sub-fields at a sequence of brightness weighting values xe2x80x981, 2, 4, 8, 16, 64, 32, 64, which was suggested in U.S. Pat. No. 6,100,939. Other example is a scheme of randomly arranging a sequence of sub-fields for each frame in accordance with an input image signal, which was suggested in Japanese Laid-open Gazette No. Pyung 7-27135. Such schemes of re-arranging a sequence of sub-fields are capable of reducing the contour noise to a certain degree. However, since it is virtually impossible for such schemes to meet all events at which any contour noise is generated because the contour noise appears in various types in accordance with an input image signal, such schemes have a limit that a contour noise reduction effect fails to reach to a desired level.
Recently, in order to eliminate a moving picture pseudo contour noise, there has been suggested a code (hereinafter xe2x80x98contour noise free codexe2x80x99) that allows all sub-fields from a sub-field arranged at an initial time of the frame until sub-fields arranged thereafter to be continuously turned on in response to an enlargement of a gray level value as shown in FIG. 4. In the contour noise free code, a brightness weighting value of each sub-field is determined in order that an emission of a light can be linearly increased, when viewed at the time axis, to thereby prevent a generation of the contour noise as shown in FIG. 5.
As can be seen from FIG. 6, the contour noise free code has a disadvantage that an expressible gray level value is limited to xe2x80x98the number of sub-fields plus 1xe2x80x99. For example, in the contour noise free code as shown in FIG. 5, a brightness weighting value of each sub-field is set to 1, 2, 4, 8, 16, 24, 32, 40, 56 and 72 to thereby limit the gray level value into 11 gray levels of 0, 1, 3, 7, 15, 31, 55, 87, 127, 183 and 255 corresponding thereto.
For this reason, a use of the contour noise free code raises a problem that though no contour noise is generated, the number of expressible gray levels becomes insufficient to deteriorate a picture quality. In order to compensate for such a reduction in total number of gray levels from the contour noise free code, a multi-toning technique using an error diffusion method, which permits to visually recognize a larger number of gray levels than the number of real gray levels, may be applied. However, the multi-toning technique brings about a deterioration of picture quality caused by an error diffusion artifact or a dithering pattern, etc.
In the mean time, a light-emission pattern determined by a combination of sub-fields is selected from a considerably large number of events. For this reason, it is virtually impossible to find out an optimal light-emission pattern capable of minimizing the contour noise from all possible light-emission patterns.
Accordingly, it is an object of the present invention to provide a method of generating an optimal light-emission pattern for a plasma display panel in order to select a light-emission pattern where a moving picture pseudo contour noise is minimized.
Another object of the present invention is to provide a method of measuring a contour noise for a plasma display panel in order to rapidly calculate a contour noise degree.
It is still another object of the present invention to provide a method of selecting a gray scale in order to select a sub-field array and a gray scale with which the contour noise is minimized.
In order to achieve these and other objects of the invention, a method of generating an optimal light-emission pattern for a plasma display panel according to one aspect of the present invention includes steps of determining a plurality of light-emission patterns with respect to an arbitrary gray level; calculating a contour noise degree between a contour noise free gray level being set in advance and the light-emission patterns given to each gray level in plurality; and selecting a light-emission pattern whose contour noise degree is minimal as a light-emission pattern with respect to an arbitrary gray level.
In the method, the contour noise degree is calculated by the sum of contour noise distance dCN defined as following equation.
dCN(Bi,Bj,SP)=|Bixe2x88x92Bj|xe2x96xa1SPxe2x88x92ixe2x88x92j|
Herein, Bi, Bj is light-emission pattern codes of gray level i and gray level j respectively, and SP is brightness weighting values of all sub-fields.
A method of measuring a contour noise of a plasma display panel according to another aspect of the present invention includes steps of determining a plurality of sub-field arrays; calculating a contour noise degree between a contour noise free gray level being set in advance and each gray level of the sub-field arrays; summing up the contour noise degree of each gray level calculated; and selecting any one among the sub-field arrays in accordance with the sum of the contour noise degree.
The method further includes a step of selecting a sub-field array, the sum of whose contour noise degree is minimal, among sub-field arrays, the sum of whose contour noise degree is calculated.
The method further includes a step of after summing up the contour noise degree of each gray level calculated, calculating an average contour noise degree per gray level of the sub-field array by dividing the sum by the total number of gray levels.
The method further includes a step of after summing up the contour noise degree of each gray level calculated with respect to at least one sub-field array that has the total number of gray levels different from that of the sub-field arrays determined in the step of determining a plurality of sub-field arrays, calculating an average contour noise degree per gray level of the sub-field array by dividing the sum by the total number of gray levels.
The method further includes a step of selecting a sub-field array whose average contour noise degree per gray level is minimal among a plurality of sub-field arrays whose average contour noise degree per gray level is calculated.
In the method, the contour noise degree is calculated by the sum of contour noise distance dCN defined as following equation.
dCN(Bi,Bj, SP)=|Bixe2x88x92Bj|xe2x96xa1SPxe2x88x92|ixe2x88x92j|
Herein, Bi, Bj is light-emission pattern codes of gray level i and gray level j respectively, and SP is brightness weighting values of all sub-fields.
A method of measuring a contour noise of a plasma display panel according to still another aspect of the present invention includes steps of determining a plurality of sub-field arrays to which brightness weighting values are given by sub-fields; calculating a contour noise degree between a contour noise free gray level being set in advance and each gray level of the sub-field arrays; dividing the contour noise degree by a threshold value set differently in a gray level scope that is not larger than a specific gray level value and a gray level scope that is not less than the specific gray level value; summing up the contour noise degree divided by the threshold value; and selecting a sub-field array, the sum of whose contour noise degree is minimal.
The method further includes a step of after summing up the contour noise degree divided by the threshold value, calculating an average contour noise degree per gray level of the sub-field array by dividing the sum by the total number of gray levels.
The method further includes a step of selecting a sub-field array whose average contour noise degree per gray level is minimal among a plurality of sub-field arrays whose average contour noise degree per gray level is calculated.
A method of selecting a gray level for a plasma display panel according to still another aspect of the present invention includes steps of determining a sub-field array to which brightness weighting values are given by sub-fields; calculating a contour noise degree between a contour noise free gray level being set in advance and each gray level of the sub-field array; comparing the contour noise degree with the threshold value being set in advance, then selecting only gray levels whose contour noise degree is smaller than the threshold value; and displaying an image only with the selected gray level.
In the method, the contour noise degree is calculated by the sum of contour noise distance dCN defined as following equation.
dCN(Bi,Bj,SP)=|Bixe2x88x92Bj|xe2x96xa1SPxe2x88x92|ixe2x88x92j|
Herein, Bi, Bj is light-emission pattern codes of gray level i and gray level j respectively, and SP is brightness weighting values of all sub-fields.
In the method, the threshold value is determined in accordance with at least any one of the amount of the contour noise degree and a gray level expression scope where it is possible to be displayed.
The method further includes a step of performing an error diffusion with respect to the gray level of the image for compensating a non-selected gray level that is bigger than the threshold value.
In the method, the threshold value is set differently in a low gray level that is not larger than a specific gray level value and in a high gray level that is not less than the specific gray level value.
In the method, the threshold value increases by a different gradient from each other respectively in a low gray level and a middle gray level that are not larger than a specific gray level value, and sustains a fixed value in a high gray level that is not less than the specific gray level value
In the method, the threshold value increases linearly in a low gray level scope where the gray level is not larger than a specific gray level value, and sustains a fixed value in a high gray level scope where the gray level is not less than the specific gray level value.
A method of selecting a gray level for a plasma display panel according to still another aspect of the present invention includes steps of determining a plurality of sub-field arrays to which brightness weighting values are given; calculating a contour noise degree between a contour noise free gray level being set in advance and each gray level of the sub-field arrays; comparing the contour noise degree with the threshold value being set in advance, then selecting only gray levels whose contour noise degree is smaller than the threshold value; and selecting a sub-field array with its frequency of use maximal in reference of the frequency of use of the selected gray level.
A method of selecting a gray level for a plasma display panel according to still another aspect of the present invention includes steps of determining a plurality of sub-field arrays to which brightness weighting values are given; calculating a contour noise degree between a contour noise free gray level being set in advance and each gray level of the sub-field arrays; comparing the contour noise degree with the threshold value being set in advance, then selecting only gray levels whose contour noise degree is smaller than the threshold value and setting a gray level that is bigger than the threshold value as a non-selected gray level; and calculating the frequency of use of the non-selected gray level, and selecting a sub-field array with its frequency of use minimal.